Circuit singulation system and method

ABSTRACT

Electronic circuits such as IC packages, circuit boards, of flex circuits are singulated by laser cutting of adjoining laminated material. The laser beam has a wavelength of less than 400 nm, and either a minimum energy density of 100 J/cm2 or a minimum power density of 1GW/cm2. The method avoids the need for cleaning and intermediate handling, and there is a greatly improved throughput.

INTRODUCTION

[0001] The invention relates to a method and machine to cut circuitboards or integrated circuit packages.

FIELD OF THE INVENTION

[0002] In recent years, in the microelectronics industry there has beena drive for lightweight low profile consumer electronics products suchas laptop computers and mobile telephones. These consumer electronicsproducts require high-throughput assembly of low-cost, low-profile andlightweight integrated circuit packages. Integrated circuits arepackaged in multiple units to achieve the required throughput and toreduce handling requirements. At the end of tis process it is thennecessary to singulate the packaged devices.

[0003] By reducing the area of the integrated circuit package it is alsopossible to reduce the area of the circuit boards on which these deviceswill perform their function. To this end, circuit boards are becominglighter and smaller.

[0004] The materials from which integrated circuit packages or circuitboards are fabricated may include, for example, copper layers, gillsfiber layers or weave, FR4, BT glass/epoxy, adhesives, encapsulants,solder masks or semiconductor. Another type of circuit board, is apolymer-based flexible (flex) circuit. Also, the invention may beapplied to cut thin layers such as liquid crystal sheets orelectrochromic dielectric thin films as used in displays.

[0005] Figs. A(a) to A(d) show examples of strips on which several BGAdevices are mounted. In Fig. A(a) encapsulant material 1 protects thedie 2 and the electrical connections between the die and the substrate3. The assembly process may require the presence of tooling holes andcut out sections 4 for ease of punching. The strip of Fig. A(a) issimilar to that of Fig. A(a) except that cut out strips are missing, andin the strip of Fig. A(c) there are no tooling holes. In the strip ofFig A(d) the encapsulant covers multiple dies. In this situation, BGAsingulation requires that the encapsulant be cut also. The dashed lines5 in the drawings indicate the cutting lines to singulate individualpackages.

[0006] A further example of a multiple unit, chip-scale package assemblyis shown in Fig. B. In this example dies are mounted on a substrate intwo dimensions to form an N×N assembly of BGA packages 6. Fig. B showsthe underside of the assembly. Solder balls 7 are positioned at thecorrect position on the circuit board and are then reflowed. Thereflowed solder balls then form the electrical contact between thecircuit board and the package. The electrical connection to the die isthrough the package substrate. The substrate layers may be copperlayers, glass fiber layers or weave, FR4, BT glass/epoxy, adhesives,encapsulants, solder masks or semiconductor. Referring to the end viewin Fig. B the substrate often comprises multiple layers which mayinclude solder mask 8, copper 9, dielectric 10, gas 11, and epoxy 12.Gold or another conductor may be used in the layer 9 instead of copper.

[0007] Fig. C shows an ample of a circuit board panel containingmultiple circuit boards 21. Such panels may be those used for “smartcards” or mobile telephone circuits. The circuit board material may berigid or flexible material made from laminated layers of copper layers,glass fiber layers or weave, FR4, BT glass/epoxy, adhesives,encapsulants, solder masks, or other materials used in circuit boardmanufacture.

[0008] The circuit board may be of a flexible material. This type ofcircuit generally is made from layers of copper, adhesive and polymersuch ad Kapton polyimide or another polymer with the required mechanicalproperties.

[0009] The electronics industry also uses liquid crystal,electro-chromic or more generally, thin film sheeting in liquid crystaldisplays or in mass-produced display assemblies.

[0010] Regarding the preset methods of singulation, Fig. D shows thefinal steps involved in the present method of manufacturing of BGA/CSPdevices. Due to the nature of the devices and systems involved, severalhanding and cleaning steps must be added in order to support thesingulation process with wafer saws. The steps include:

[0011] electrical test and laser mark in boats, panels or trays,

[0012] removal from boat or tray and mounting on tacky UV tape,

[0013] cutting with saw and cleaning,

[0014] UV cure,

[0015] placing on trays,

[0016] marketing and inspection.

[0017] Sawing and punching of chip scale packages is described inWO9903129: (Singulation system for chip scale packages) and inWO98/52212. (Pick and place cutting head that separates chip scalepackages from a Multi layered film strip). Several consumables, such asUV tape, wafer rings, saw blades, and cleaning solutions must be used.

[0018] An object of the invention is to provide a system and a method tocut through the above materials at a rate sufficient to meet thesingulation rate requirements for a production line.

[0019] Another object is that the method and apparatus provide a higheryield by reducing the extent of deposited debris and by reducinghandling requirements.

SUMMARY OF THE INVENTION

[0020] According to the invention, there is provided a method forsingulating an electronic circuit by cutting laminated material joiningthe circuits, the method comprising the step of:-

[0021] generating a laser beam having the flowing properties:

[0022] a wavelength of less than 400 nm, and

[0023] a minimum energy density of 100 J/cm² or a minimum peak powerdensity of 1GW/cm²;

[0024] aligning the beam relative to a feature or fiducial of thematerial; and

[0025] training the beam along the material until a cut has been made.

[0026] In one embodiment, beam is moved to have a spatial overlap ofconsecutive pulses, the overlap being in the range of 5% to 95%.

[0027] In one embodiment, the overlap is in the range of 30% to 50%.

[0028] In another embodiment, the beam is moved in a plurality ofpasses.

[0029] In a further embodiment, the beam is moved in greater than fivepasses.

[0030] In one embodiment, the beam is generated with a pulse repetitionrate of greater than 1 kHz.

[0031] In one embodiment, the thickness of the laminated structures maybe up to the thickness defined by the depth of focus of the laser beam.

[0032] In one embodiment, the laminate material contains two or morelayers selected from BT epoxy, glass fibers, copper, gold, poly-imide,adhesive, overmold materials, underfills, conductors, dielectrics,stiffeners, stabilisers, protectors or other materials as used inelectronic packaging.

[0033] In another embodiment, the individual layers of the laminatematerial have different ablation and ionization thresholds, differentabalation and ionization rates, and different non-linear absorption andnon-ionization coefficients.

[0034] In a further embodiment, the beam is generated from a solid statelaser with a characteristic average power peak at a specific repetitionfrequency.

[0035] In one embodiment, the beam is controlled so that the averagepower drop as the repetition frequency is increased or decreased, andalthough individual pulse energy may be increased at a repetitionfrequency other than the repetition frequency for maximum average powerthe maximum cut rate is achieved at a repetition frequency other thaneither of these frequencies due to the contribution of other lasercutting parameters.

[0036] In one embodiment, the average power of said laser beam isgreater than 3W, with a pulse width less than 100 nanoseconds, aconsecutive pulse spatial overlap of 10-70%, and a beam diameter lessthan 70 microns at the 1/e² point of a spatial intensity profile.

[0037] In one embodiment, the laser beam is generated by a diode laserpumped gain medium device with a fundamental emission in the 900 to 1600nm wavelength range and with second, third, fourth or fifth harmonicgeneration of {fraction (1/2)}, {fraction (1/3)}, {fraction (1/4)},{fraction (1/5)}^(th) of this wavelength which is obtained by placingappropriate crystals in the laser cavity or outside the laser cavity.

[0038] In one embodiment, said laser device may be of the Nd:YAG,ND:YLF, Nd:YVO4 or the other combinations of Impurity:Host gain medialasting in the required range and with harmonic generation to anoperating wavelength of less than 400 nm.

[0039] In one embodiment, the beam is delivered to the work surfaceusing one or more mirrors mounted on one or more scanning galvanometers,and in which the required spot size is achieved by use of an on-axislens position adjustment at a stage before the galvanometer mirror, andat a stage after the galvanometer mirror by a lens of a flat field lens,or by the use of a combination of these lenses.

[0040] In one embodiment, the laser beam is delivered using one or moremirrors mounted on one or more translational stages, and focusing isachieved by the use of a telescope or an on-axis lens before the movingmirrors or lens mounted before the sample surface and moving with thebeam delivery mirror such that the focussed beam is delivered to thesample surface.

[0041] In another embodiment, the beam is telescoped and focussed toachieve the required spot size at the cutting plane with the telescopeor scan lens chosen such that the beam waist remains within a specifiedpercentage of the optimum spot size throughout the range over which thebeam is delivered, and where the range is greater than the thickness ofthe part

[0042] In a further embodiment, an assist gas is used to assist thecutting process to prevent debris from being deposited on the materialsurface, and wherein the assist gas removes material generated duringthe cut process so that it does not create absorption or scattering ofconsecutive laser light pulses.

[0043] In a still further embodiment, the assist gas is used to providean inert atmosphere to prevent unwanted specific photochemical orphoto-physical reactions form occurring during cutting.

[0044] In one embodiment, a vacuum suction process is used to extractfumes and solid debris generated at the cut surface.

[0045] In one embodiment, alignment of the laser beam to a feature onthe material surface is achieved by use of a sensor and means for imageprocessing to provide the coordinates along which cutting occurs, andwherein a beam positioning mechanism is controlled to ensure that thelaser beam follows the required cutting path.

[0046] According to another aspect the invention provides a circuitsingulation system comprising:

[0047] means for supporting a set of electronic circuits interconnectedby material;

[0048] a laser beam source comprising means for generating a laser beamhaving:

[0049] a wavelength of less than 400 nm, and

[0050] a minimum energy density of 100J/cm² or a peak power density of1GW/cm²,

[0051] a beam positioning mechanism comprising means for directing thebeam a the material and for training it along cut lines to singulateelectronic circuits.

[0052] In one embodiment, the beam positioning system comprises a seriesof mirrors, at least some of which are movable for directing the laserbeam, and a focusing lens.

[0053] In one embodiment, the mirrors are linearly movable.

[0054] In another embodiment, the mirrors are rotatable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawing in which:-

[0056]FIG. 1 is a diagrammatic view of a cutting method of theinvention;

[0057]FIGS. 2 and 3 are diagrams showing control of a laser beam forcutting;

[0058]FIGS. 4 and 5 are plots indicting cutting performance; and

[0059] FIGS. 6 to 9 inclusive are photographs of cut materialcross-sections.

BRIEF DESCRIPTION OF THE INVENTION DESCRIPTION OF THE EMBODIMENTS

[0060] The invention is based upon use of a laser beam for singulation,as shown in FIG. 1. The process uses an ultraviolet laser to “saw” thepackage. The laser creates excellent quality cuts which generate onlyvapor debris, use no consumables, and do not cause any chips ormicro-cracks in the cut edges.

[0061] The concept of photo-ablative decomposition of materials usingultraviolet laser pulses was introduced by Srinivasin in 1982. (R.Srinavasan et al. Applied physic letters, 41, page 576, 1982). The basicmechanism for material removal differs from longer wavelength processesin that the energy of individual photons at ultra-violet wavelengths isclose to the energy of the bonds that hold the material together.Examples of the covalent bond strengths of some representative bonds areshown in Table 1. In addition, at room temperature, the width of theabsorption bands is sufficient that, at least for organic covalentbonds, pulsed ultraviolet radiation in the wavelength range below 400 nm(>3eV) is sufficient to cause photo-ablative decomposition. TABLE 1Covalent bond strength iii polyatomic organic compounds (Source:handbook of chemistry and physics page F239, Ed. 1978-1979, CRC press).The corresponding wavelength of a photon with this energy is also shown.Bondtype eV λ H₃C-CH₃ 3.83 324 H—CH₃ 4.53 274 CH₃CO—COCH₃ 2.93 423C₆H₅CH₂—N(CH₃)₂ 2.66 466 F—CH₃ 4.70 264

[0062] From our investigations into photo-ablative decomposition thefollowing conclusions may be drawn:

[0063] Below a certain threshold of fluence or intensity, no ablation isobserved. However, photo-degradation can occur and lead to morphologicalchanges on the material surface. (P. E.Dyer et al, Journal of appliedphysics, 57. p1420, 1985; J. H. Brannon et al., Journal of appliedphysics, 58, p2036, 1985; R. Srinivasan et al., Applied physics 61 p372,1987)

[0064] The fragments coming off the surface that is being ablatedconsist of small clusters of material and gaseous products. (R. C.Estleret al, Applied physics letters, 49, p1175, 1986; G. Koren et al, Appliedphysics letters, 44. p1112, 1984; G. S. Hansen et al., Journal ofapplied physics, p1878, 1990.)

[0065] With the exception of 308 nm ablation of polyimide (D. L.Singleton et al., Chemical physics, 144, p415, 1990), the laser pulseduration and shape are reported to have a determinant effect on theablation. (R. Srinivasan et al, Applied physics letters, 53,p 1233,1988.)

[0066] The photo-ablation process generally occurs on a fast time-scaleless than the laser pulse width of 20 ns.

[0067] A blast wave is generated following pulse impact and absorption.(R. Srinivasan et al, Journal of applied physics, 68, p1842, 1990).

[0068] Absorption coefficient is defined at low power density (mW/cm2)and etch raw represents material removal at much higher power densities(mW to MW/cm2). As ablation starts with absorption, there is often alink between absorption coefficient, alpha, and the etch depth. That is,a strong absorber will have a lower ablation threshold (power density)than a weak absorber. However, etch rate for a strong absorber can bemuch smaller as most of the photonic energy is confined to shallowerdepths. This is a common observation. However, this is by no means thegeneral case. Absorption is only a necessary condition. It is notsufficient. A high absorber could relax radiativley or thermally to itsoriginal ground state. To have photo-ablation requires bond breaking,i.e. a reaction from reactant to products via excited state(photo-chemical reaction) or ground state (photo-thermal reaction).Secondly, because of the different intensity regime between thespectroscopic measurement of alpha and the ablation regime, non-linearphenomena such as multi-photon absorption could lead to a dramaticincrease (and then possibly to saturation) of the etch rate with powerdensity. In this way, two materials having a similar alpha (i.e. at lowintensity) but completely different etch rates (at high intensity)because, in one material, two-photon or multi-photon absorption sets in.For example, at 355 nm the aromatic rings of organic are not yet excited(absorption peaks at 240-280 nm). But with sufficient power-photon bondbreaking may occur.

[0069] A true description of the processes is difficult because (i) mostof the cross sections and relaxation times are unknown, and (ii)ablation happens in the solid state where multi-molecular events, (i.e.the way the energy of the broken bonds dissipates into the matrix),mechanical properties, and thermal properties are also of importance.

[0070] The apparatus of the invention uses an ultra-violet solid statelaser to cut materials that form chip scale packages such as ball gridarrays, circuit board materials and other “thin film” type materialsthat may form the lamellae of such laminated packages.

[0071] Solid state YAG/YLF and NVO4 lasers and other lasers operatingwith fundamental laser emission in the visible or infra red may beconverted to ultraviolet lasers through second and third harmonicgeneration, sum and difference frequency mixing in second ordernonlinear crystals.

[0072] As an example, using lasers operating in the “third harmonic”regime we have established that with average power in excess of 3W, wecan meet or better the cut rate required to match most BGA process linethroughputs.

[0073] The apparatus consists of an ultraviolet laser system asdescribed above, and a beam positioning mechanism. The bean positioningmechanism may be a multiple axis stage as illustrated in FIG. 2 or ascanning galvanometer as illustrated in FIG. 3.

[0074] Referring to FIG. 2 a beam positioning mechanism 30 isillustrated. A frame 31 supports a target 32 to be singulated. Theoptical path is:-

[0075]33: laser beam source,

[0076]34: beam optics (expander),

[0077]35: fixed mirror,

[0078]36: fixed mirror,

[0079]37: mirror movable in X direction,

[0080]38: mirror movable in Y direction,

[0081]39: camera for alignment to package features, and

[0082]40: a focusing lens.

[0083] The advantages of the translational stage approach are;

[0084] Small spot sizes are possible over a large area

[0085] Variation in spot size may be effectively eliminated by mirrorcontrol.

[0086] Highly repeatable and accurate positioning

[0087] The cut process may be monitored in real time by observing thelight energy reflected from the surface of the material being cutback-reflected along the path of the incident beam and on to aphoto-electric detector. In the event that the material has beencompletely cut, there will be no reflective surface and the signal atthe detector will drop to a background level, ideally zero. Thereflected light may be U.V. laser light, in which case a beam splitterfor the laser wavelength is used. In another embodiment a mirrortransparent to Ultraviolet (e.g. 355 nm) is used to reflect a secondwavelength such as the second harmonic wavelength (e.g. 532 nm). In thisinstance, there is no input power sacrifice at the ultravioletwavelength.

[0088] Another possibility is that the feedback mechanism is the visibleto infrared emission of the plume generated during the cutting process.In the absence of this plume, no cutting occurs and the sample has beencut through. The presence of the plume may be taken as evidence thatcutting is occurring. Furthermore, the spectral content of the plumeprovides information on the material that is being cut.

[0089] Real time control of cutting multi-layer materials is possible bymonitoring the spectral output at each position of the pulse on the cutscan. From the control point, once the signal can be delivered andprocessed before the next measurement occurs it is possible to controlthe laser output parameters on a pulse to pulse basis. This isbeneficial in the case where composite materials such as laminatedpackages may have variation in material distribution across the cutsurface. In addition, once a layer has been completely cut through thelaser parameters can be vaned on the by such that the cut process isoptimised for the next layer.

[0090] A second option for beam delivery and position control is to usea scanning galvanometer. FIG. 3 shows the essential geometry. Two minorsare mounted on current driven galvanometer coils that rotate the axes onwhich the mirror is mounted. Scanning from position A to position D canbe achieved by scanning galvanometer Galv.1 through an angle G1 withgalvanometer Galv.2 oriented so that its mirror reflects along thisline. Scanning from position A to position B may be achieved by rotatinggalvanometer Galv.2 through an angle G2 with galvanometer Galv.1 fixedfor position A on the AD line. Accordingly, any position in twodimensions may be marked out by a combination of moves of the twomirrors. The galvanometer approach allows high acceleration anddeceleration times, resulting in minimal time between moves and minimaleffect on processing time.

[0091] To deliver a focussed laser beam to the work surface it isnecessary to use a flat field lens or a z-axis focus adjustment. Bothgalvanometer and gantry systems are possible for ultraviolet laserexcising of circuit board and chip scale package materials. The choicedepends on the exact requirements of the cutting process.

[0092] In a situation where the high acceleration of a galvanometerapproach is required with high accuracy a combination of a galvanometerand XY stage or gantry approach can provide this. For example, aspecimen may be mounted on an XY stage below a galvanometer arrangement.High speed and acceleration is possible over a small area with a highdegree of accuracy through the galvo action and slower moves with highaccuracy are possible with the XY stage. Where the number of slowermoves is low, the overall effect on processing time will be negligible.This galvo/XY stage combination allows accuracy and speed to beexploited.

[0093] The type of laser, the laser output specifications, the beamdelivery method and the material that is to be cut all affect the rateat which the material is cut and the quality of the resulting cut.

[0094] The optical configuration described above delivers a laser beamto the surface to be cut. Using a lens, the beam is focussed to a smalldiameter, high-intensity spot. The laser fires pulses in sequence atrates greater than 1 kHz. Cutting is achieved by scanning the beam overthe sample (or scanning the sample through the beam) at a speed suchthat there is sufficient overlap between the pulses to allow cutting. Wehave established that in order to maximise the cut speed and quality itis necessary to ensure the correct overlap factor and to use multiplepasses over the same path to cut through a material. From ourinvestigation into the cutting process we have established that;

[0095] The laser, material and scanning parameters form a set ofparameters that must be optimised for the optimum cut speed and quality.

[0096] Using the average power, repetition rate, and scan raterelationships it is possible to ensure a linear scaling of cut rate withaverage power by using multiple passes.

[0097] The cut quality is also affected by the overlap.

[0098] The focal spot size may be reduced or increased and thecorresponding overlap parameter adjusted (by adjusting rep rate or scanrate) if higher peak power density is required.

[0099] The high power density (>100MW/cm2) scaling laws for etch rate orcut rate are different to those observed for low power (1KW to100MWY/cm2)

[0100] Parameter Space

[0101] (a) Laser Parameters

[0102] Wavelength of the laser beam.

[0103] Pulse energy: The pulse is the energy within a single pulse ofthe pulse train, and the energy density is the value per cm²

[0104] Repetition frequency defines the number of pulses per second.

[0105] Scan speed: The scan speed of the laser relative to he sample isdefined by the required spatial overlap for a given spot size at a fixedrepetition rate.

[0106] Overlap: The overlap is defined as the percentage of the beamdiameter that overlaps spatially with the other pluses in sequence.

[0107] Spot size: The spot size or bean diameter determines the peakpower density(W/cm2) and fluence (J/cm2) at the cutting surface. Also,the spot size determines at which velocity the laser should be scannedrelative to the sample to achieve the required overlap.${{Overlap}\quad (\%)} = {\frac{{{Beam}\quad {diameter}} - \left\{ \frac{{scan}\quad {velocity}}{{repetition}\quad {frequency}} \right\}}{Diameter} \times 100}$

[0108] Average power is the pulse energy delivered by all pulses in onesecond. Energy ×Repetition frequency.${{Peak}\quad {Power}} = \frac{Energy}{pulsewidth}$

[0109] Pulse width: Pulse width determines the peak power and the peakpower density${{Peak}\quad {Power}\quad {Density}} = \frac{{Peak}\quad {power}}{Area}$

[0110] Peak power density is the peak power delivered per unit area(cm²).

[0111] (b) Material parameters

[0112] Absorption coefficient

[0113] The linear absorption coefficient α, determines the penetrationdepth of the pulse. The Beer Lambert law states that

[0114]I=I ₀ e ^(−αL)

[0115] Accordingly, the depth at which the incident intensity, I₀,reduces by 50% (I=I₀/2) is given by$L_{\frac{1}{2}} = \frac{Ln2}{\alpha}$

[0116] Two-photon absorption way be understood by taking the totalabsorption as the sum of a linear and intensity dependent absorptioncoefficient given by

[0117] α(I)=α₀+α₂ I

[0118] where α₀ is the linear absorption coefficient in m⁻¹, α₂ is thetwo-photon absorption coefficient in m/W and I is the intensity in W/m²

[0119] For a fixed intensity wave and at a wavelength where two-photonabsorption occurs the resulting decrease in the wave intensity may berepresented mathematically by $\begin{matrix}{\frac{{I(z)}}{z} = {{{- \alpha_{0}}{I(z)}} - {\alpha_{2}{I^{2}(z)}}}} & B\end{matrix}$

[0120] where z is the distance that the wave has propagated in meters.Integration of this expression yields the intensity of the wave as afunction of the distance propagated; $\begin{matrix}{{I(z)} = \frac{I_{0}^{{- \alpha_{0}}z}}{\frac{1}{\alpha_{0}}\left\lbrack {\alpha_{0} + {\alpha_{2}{I_{0}\left( {1 - ^{{- \alpha_{0}}z}} \right)}}} \right\rbrack}} & C\end{matrix}$

[0121] where I₀ is the initial intensity of the wave. If there isnegligible two-photon absorption, α₂, is zero and this expressionreduces to the Beer-Lambert law for linear absorption. In the presenceof two-photon absorption an intensity limiting effect occurs.

[0122] Considering these expressions the following conclusions may bedrawn.

[0123] Energy may be absorbed even if the low intensity absorptioncoefficient is zero. (A)

[0124] As the intensity is increased the total energy absorbed increases(A)

[0125] As Intensity is increased in the presence of two photonabsorption, the depth at which the intensity is above a threshold value,I_(th), increases also.

[0126] Multi-photon absorption:

[0127] Similar to two photon absorption three and multi-photonabsorption can be defined using the procedures above.

[0128] Nonlinear absorption:

[0129] Other nonlinear absorption processes include absorptionsaturation, inverse saturable absorption, and photo-ionisation.

[0130] Using an assist gas it is possible to ensure that debris is notredeposited on thc sample surface. Also, some gases may enhancephoto-removal of material and others may suppress unwanted non-cleanchemical processes.

[0131] Cut Rate Scaling Parameters

[0132] Based on the parameter space described it is possible to statethe following.

[0133] The wavelength should be less than 400 nm.

[0134] The minimum energy density should be 100J/cm² or the minimum peakpower density should be 1.0GW/cm².

[0135] For a given laser pulse energy, the scan rate and repetition ratedefine overlap. By increasing scan rate and repetition rate the overlapcan be maintained, the number of scans remains constant but the timerequired to cut decreases linearly according to the increase in scanrate, i.e. the cut rate decreases linearly.

[0136] For a given pulse repetition frequency, scan speed and overlap(5% -95°), the number of passes required to cut through a sampledecreases as the energy of the pulse is increased. For UV solid starelasers with average power up to 4W working at the optimum cut rate andwith parameters set to achieve is, the increase generally follows alinear trend. As well as standard photo-ablation, nonlinear absorptionand ionization effects occur in the higher power density regime (greateran 100MW/cm²).

[0137] Accordingly, the cut rate increases with a linear or better thanlinear dependence on pulse energy. For the UV lasers described above, itis possible to reduce the number of scars required to achieve cuttingby:

[0138] optimising the cut rate at a low power of 3 W. and

[0139] by scaling the average power output upward such that the onlyparameter to change is the pulse energy (for example yielding 15Waverage power),.

[0140] Depending on the material it may be possible to reduce the numberof scans by five (15W/3W, linear), more the five (super-linear) or lessthan five (saturating) to achieve complete cutting. Generally, for thematerials in chip scale packages, circuit boards and flex circuits, thecut rate increases linearly or “super” linearly as pulse energy isincreased.

[0141] As example of the scaling laws above consider a 1 mm thickmulti-layer structure. We achieve a cut rate of 4.2 mm/s with an averagepower of 3W. This is achieved with a repetition rate of 5.5 kHz,pulse-width of 95 ns, scanning at 100 mm/s and with a 25 micron spotsize. This requires a total of 24 passes to cut through and achieve thequality required. For this example the corresponding energy density (orfluence) and peak power density may be calculated as:${{Average}\quad {Power}\quad {Density}} = {\frac{Energy}{Area} = {\frac{{Average}\quad {Power}}{{repetition}\quad {rate} \times {Area}} = {111{J/{cm}^{2}}}}}$${{Peak}\quad {Power}\quad {Density}} = {\frac{{Peak}\quad {power}}{Area} = {\frac{{Average}\quad {Power}}{{repetition}\quad {rate} \times {pulse}\quad {width} \times {Area}} = {1.2\quad G\quad {W/{cm}^{2}}}}}$

[0142] Increasing the scan rate to 300 mm/s and increasing therepetition rate to 16.5 kHz, and taking the same pulse energy, it ispossible to achieve a cut rate of at least 12.6 mm/s. This requires anaverage power of 9W. This is within the capability of diode pumped UVsolid state lasers.

[0143] Increasing the power further at this repetition rate (for exampleto 18W) and maintaining all other parameters can then result in adoubling of this rate to 25.2 mm/s.

[0144] Keeping all of the above parameters the same, furtherimprovements (cut rate increases) are possible by reducing thepulse-width. Reducing pulse-width for a fixed energy results in anincrease in peak power. Nonlinear processes are strongly dependent onpeak power and where there is a nonlinear absorption, ionization, orrefraction process contributing to the rate of material removal,reducing the pulse-width will increase the material removal rateresulting in an increased cut rate, Ablation also show a strong pulsewidth dependence.

[0145] A further parameter that affects the beam energy/power density atthe sample is the beam diameter. Focussing to a smaller beam waist atthe sample results in an increase in the peak power density, averagepower density and energy density at te sample. All of these result in anincrease in material removal. Eventually, however the “kerf” or width ofthe cutting region physically occludes material removal and it isnecessary to ensure a sufficient kerf width to maintain the materialremoval rate. A second factor which much be taken into account is thedepth of field. Focussing too tightly will reduce the depth of field.Finally, reproducibility of the positioning system will become criticalat smaller beam sizes. With these factors in mind, the optimum beamdiameter lies in the range from 8 to 70 microns.

[0146] The parameter space is then defined by (in no particular order):

[0147] Pulsewidth: <100 ns

[0148] Beam waist: <70 microns

[0149] Average power: >3W

[0150] Repetition frequency >1 kHz

[0151] Number of passes: 1

[0152] Wavelength: <400 nm

[0153] Overlap (rep rate and scan rate dependent): 5% to 95%, andpreferably 30% to 50%.

[0154] Minimum energy density of 100J/cm² or a minimum peak powerdensity of 1.0GW/cm².

EXAMPLES Example 1 Flex Circuit Packages and Liquid Crystal Sheet

[0155] The laser was a frequency tripled Q-switched Nd: YAG laser. Thelaser stability was better than 7% about the mean and the mode rationwas in the fundamental TEM₀₀. The operating specification is shown inTable 2. TABLE 2 Laser parameters for Example 1. Parameter ValueWavelength 355 nm Average power 4 W Pulse-width 95 ns Rep Rate 5.5 kHzPulse energy 0.727 mJ Fluence 148 J/cm2 Peak Power 7.7 KW Peak Powerdensity 1.56 GW/cm²

[0156] The cutting parameters for five representative flex circuitmaterials were used. The samples consist of solder mask, poly-imide andadhesive in various laminations.

[0157] Sample #1: Contains 5 layers of material. 2-poly-imide,2-adhesive, 1-solder mask Sample #1A: Sample is identical to sample # 1except for the continuation of the solder mask over the heat bond areaon one side.

[0158] Sample #2: Sample contains 3 layers of material. 1-poly-imide,2-solder

[0159] Sample #3: Sample contains 5 layers in the non-copper loadedareas. 3-poly-imide, 2-adhesive

[0160] Sample #4: Sample contains 3 layers of material(1-liquid crystalpolymer, 2-solder mask

[0161] The optimum rep rate was established by varying the rep rate andscan speed, ensuring similar overlap and counting the number of passesrequired to machine through. Although the rep rate could be tuned up to15 kHz the optimum machining performance is obtained when the averagepower is near the peak. At this point the individual pulse energy maynot be at a maximum but the total delivered energy is such that thatcutting occurs in the fastest possible time

[0162] The conclusion that may be drawn from the graphs in FIGS. 4 and 5is:

[0163] Above a power density of 0.5 GW/cm2, by varying the pulse energyonly, the cut rate increases without saturation. This relationshipvaries slightly depending on the sample type and thickness but therelationship may be defined as linearly proportional or better.

Example 2 Over Molded Ball Grid Array Package The Effect of Overlap

[0164] Using the same laser operating parameters as in Example 1 the cutrate for a 1 mm thick overmolded BGA package was measured as 4.2 mm/secat 3W average power. This was accomplished by using 24 passes at a spedof 100 mm/s ( approx. 50% overlap at the FWHM intensity point). 20X and100X SEM images of the edges obtained are shown in FIGS. 6 and 7respectively. The sample consists of solder mask, glass fiber matrix,epoxy and adhesive, copper and encapsulant regions. It is clear that theedge quality is defect free. The open space that appears is likely dueto the free volume of the encapsulant material.

[0165] The experiment was repeated with two passes and scanning with aspeed of 8.3 mm/sec. This corresponds to a 93% overlap for a 25 micronspot. The 20X and 100X SEM images of the edge are shown in FIGS. 8 and 9respectively. It is very clear that the cut quality is not as good as inthe case for 50% overlap and that this is an important factor inachieving the required cut quality.

[0166] The following example indicates the laser beam powerrequirements. 1 mm thick multilayer overmolded BGA package (see Example2).

[0167] Average power used=3W

[0168] Repetition frequency 5.5 kHz

[0169] Spot size 25 microns (1/e²)

[0170] Scan speed 100 mm/s

[0171] Number of passes required to cut through=24

[0172] Effective cut rate=4.2 mm/second

[0173]${{Energy}\quad {density}} = {\frac{Averagepower}{{repetition}\quad {frequency}*{area}} = {\frac{3}{5500*\pi*\left( {12.5 \times 10^{- 4}} \right)^{2}} = {111{J/{cm}^{2}}}}}$${{Energy}\quad {density}} = {\frac{Averagepower}{{repetition}\quad {frequency}*{area}} = {\frac{3}{5500*\pi*\left( {12.5 \times 10^{- 4}} \right)^{2}} = {111{J/{cm}^{2}}}}}$

[0174] Depth of Focus:${\omega (z)} = {\omega_{0}\left\lbrack {1 + \left( \frac{\lambda \quad z}{\pi \quad \omega_{0}^{2}} \right)^{2}} \right\rbrack}^{\frac{1}{2}}$

[0175] Where w(z) is beam waist at a distance z from the minimum beamwaist at w₀ and λ is the wavelength. Allowing 5% spot size variation,i.e. w(z)=1.05 w₀.

[0176] With w₀=25 microns and at a wavelength of 355 nm and solve forz=Δz${\Delta \quad z} = {\pm \frac{0.32{\pi \left( {25 \times 10^{- 6}} \right)}^{2}}{355 \times 10^{- 9}}}$Δ  z = ±1.7  mm

[0177] Form this data we can conclude that the energy density, peakpower density and average power density regime is unique and also thatthe depth of focus over which this regime can exist yields a completelynew set of parameters for laser machining using ultraviolet light.

[0178] Using a laser of this type with an average power greater than 3Wyields a completely new regime of power densities which may be appliedto the efficient machining of multiple layer and composite materialssuch as those used in electronic manufacturing.

[0179] It will be appreciated from the above that the invention achievesthe following advantages over the prior art.

[0180] 1. Speed Improvement:

[0181] Parts transferred directly from previous process in panel, strip,or boats

[0182] For example, for singulation of a package having a thickness of 1mm, a speed of greater than 100UPH (units per how) is achieved with a 5Wlaser. This may be greater with more power.

[0183] No cleaning is required

[0184] 2. Cost Reduction Over Existing Saws

[0185] Unlike use of a diamond saw, the laser cutting process does notincur costs associated with wafer saw, for UV tape deionized water,drying, and for saw consumables

[0186] 3. Space

[0187] Laser excising consumes less space in class 1000 clean room arm.Laser excising and inspection require only approximately 2 square metre,as opposed to approximately 5 square metres for prior art sawing,handling, and inspection equipment.

[0188] 4. Cut Quality

[0189] The excising method of the invention results in much smaller“kerf”. Typically in the prior art methods saws are given 250μm wastearea and use 175um cuts. In the invention, the waste area is 25μm orless. Thus, the invention achieves great “real estate” for components,particularly small components. Also, the invention achieves edges withless stress and no micro-cracking.

[0190] The invention is not limited to the embodiments described but maybe varied in construction and detail within the scope of the claims.

1. A method for singulating an electronic circuit by cutting laminatedmaterial joining the circuits, the method comprising the steps of:-generating a laser beam having the following properties a wavelength ofless than 400 nm, and a minimum energy density of 100 J/cm² or a minimumpeak power density of 1GW/cm²; aligning the beam relative to a featureor fiducial of the material; and training the beam along the materialuntil a cut has been made.
 2. A method as claimed in claim 1, whereinthe beam is moved to have a spatial overlap of consecutive pulses, theoverlap being in the range of 5% to 95%.
 3. A method as claimed in claim2, wherein the overlap is in the range of 30% to 50%.
 4. A method asclaimed in claim 1, wherein the beam is moved in a plurality of passes.5. A method as claimed in claim 4, wherein the beam is moved in greaterthan five passes.
 6. A method as claimed in claim 1, wherein the beam isgenerated with a pulse repetition rate of greater than 1 kHz.
 7. Amethod as claimed in claim 1, wherein the thickness of the laminatedstructures may be up to the thickness defined by the depth of focus ofthe laser beam.
 8. A method as claimed in claim 1, wherein the laminatematerial contains two or more layers selected from BT epoxy, glassfibers, copper, gold, poly-imide, adhesive, overmold materials,underfills, conductors, dielectrics, stiffeners, stabilisers, protectorsor other materials as used in electronic packaging,
 9. A method asclaimed in a claim 1, wherein the individual layers of the laminatematerial have different ablation and ionization thresholds, differentabalation and ionization rates, and different non-linear absorption andnon-ionization coefficients.
 10. A method as claimed in claim 1, whereinthe beam is generated from a solid state laser with a characteristicsaverage power peak at a specific repetition frequency.
 11. A method asclaimed in claim 1, wherein the beam is controlled so that the averagepower drops as the repetition frequency is increased or decreased, andalthough individual pulse energy may be increased at a repetitionfrequency other than the repetition frequency for maximum average powerthe maximum cut rare is achieved at a repetition frequency other thaneither of these frequencies due to the contribution of other lasercutting parameters
 12. A method as claimed in claim 1, wherein theaverage power of said laser beam is greater than 3W, with a pulse widthless than 100 nanoseconds, a consecutive pulse spatial overlap of10-70%, and a beam diameter less than 70 microns at the 1/e² point of aspatial intensity profile.
 13. A method as claimed in claim 1, whereinthe laser beam is generated by a diode laser pumped gain medium devicewith a fundamental emission in the 900 to 1600 nm wavelength range andwith second, third, fourth or fifth harmonic generation of {fraction(1/2)}, {fraction (1/3)}, {fraction (1/4)}, {fraction (1/5)}^(th) ofthis wavelength which is obtained by placing appropriate crystals in thelaser cavity or outside the laser cavity.
 14. A method as claimed inclaim 13, wherein said laser device may be of the Nd:YAG, ND:YLF,Nd:YVO4 or the other combinations of Impurity:Host gain media lasting inthe required range and with harmonic generation to an operatingwavelength of less than 400 nm.
 15. A method as claimed in claim 1,wherein the beam is delivered to the work surface using one or moremirrors mounted on one or more scanning galvanometers, and in which therequired spot site is achieved by use of an on-axis lens positionadjustment at a stage before the galvanometer mirror, and at a stageafter the galvanometer mirror by a lens of a flat field lens, or by theuse of a combination of these lenses.
 16. A method as claimed in claim1, wherein the laser beam is delivered using one or more mirrors mountedon one or more translational stages, and focusing is achieved by the useof a telescope or an on-axis lens before moving mirrors or lens mountedbefore the sample surface and moving with the beam delivery mirror suchthat the focussed beam is delivered to the sample surface.
 17. A methodas claimed in claim 1, wherein the beam is telescoped and focussed toachieve the required spot size at the cutting plane with the telescopeor scan lens chosen such that the beam waist remains within a specifiedpercentage of the optimum spot site throughout the range over which thebeam is delivered, and wherein the range is greater than the thicknessof the part.
 18. A method as claimed in claim 1, wherein an assist gasis used to assist the cutting process to prevent debris from beingdeposited on the material surface, and wherein the assist gas removesmaterial generated during the cut process so that it does not createabsorption or scattering of consecutive laser light pulses.
 19. A methodas claimed in claim 18, wherein the assist gas is used to provide aninert atmosphere to prevent unwanted specific photochemical orphoto-physical reactions form occurring during cutting.
 20. A method asclaimed in claim 1, wherein a vacuum suction process is used to extractfumes and solid debris generated at the cut surface.
 21. A method asclaimed in claim 1, wherein alignment of the laser beam to a feature onthe material surface is achieved by use of a sensor and means for imageprocessing to provide the coordinates along which cutting occurs, andwherein a beam positioning mechanism is controlled to ensue that thelaser beam follows the required cutting path.
 22. A circuit singulationsystem comprising: means for supporting a set of electronic circuitsinterconnected by material; a laser beam source comprising means forgenerating a laser beam having: a wavelength of less than 400 nm, and aminimum energy density of 100J/cm² or a peak power density of 1GW/cm², abeam positioning mechanism comprising means for directing the beam a thematerial and for training it along cut lines to singulate the electroniccircuits.
 23. A circuit singulation system as claimed in claim 22,wherein the beam positioning system comprises a series of minors atleast some of which are movable for directing the laser beam, and afocusing lens.
 24. A circuit singulation system as claimed in claim 23,wherein the mirrors are linearly movable.
 25. A circuit singulationsystem as claimed in claim 24, wherein the mirrors are rotatable.